The present invention relates to a process of using a unique hydrotalcite material as a binder for different inorganic materials such as oxide, hydroxide, spinels, and zeolites to form extrudates, spheres, or tablets which are stable and possess good mechanical strength. These inorganic materials can then be used in their respective applications as catalysts, catalyst supports or adsorbers.
In many industrial applications involving fluid-solid contacting, the catalysts or adsorbers are loaded into a fixed bed reactor in preformed shapes. The formed shapes offer the advantages of lower pressure drop and efficient fluid distribution. In these applications it is beneficial to prevent the breakup and degradation of the formed shapes under the specific process conditions. Otherwise, the result will be an increase in the pressure drop and channeling which leads to inefficient contacting. Usually the forming of an active component to be used as a catalyst or adsorber is accomplished with the aid of a binder.
Choosing the binder to be used is important since the binder itself usually contributes to the reaction. The most commonly used binders are pseudoboehmite and cationic clays such as bentonite. These materials are fairly easy to extrude and provide extrudates having excellent physical strength. Pseudoboehmite, upon calcination, converts to a gamma alumina which is a well-known acidic support. Bentonite, and other such clays, are also known to catalyze reactions via the acidic pathway. In many cases it is crucial to suppress the undesirable side reactions catalyzed by the acidic nature of the support by adding alkaline components. The acidity generated by the binder can lead to cracking reactions in catalytic applications for hydrocarbon conversion. This can lead to coking of the catalyst, and decreased catalyst cycle length (see U.S. Pat. No. 5,182,242). In such cases, a binder with neutral or basic properties can inhibit undesirable side reactions.
Hydrotalcite is a naturally occurring mineral having the formula: EQU Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3. 4H.sub.2 O
Hydrotalcite-like materials or anionic clay minerals have similar structures and have the general formula:
[M.sup.II.sub.1-x M.sup.III.sub.x ](OH).sub.2..sup.x /.sub.y A.sup.y-.mH.sub.2 O
where M.sup.II and M.sup.III are divalent and trivalent cations, respectively, and A is an anion. These materials belong to the pyroaurite-sjogrenite class of minerals and their crystal structure has been described in the literature (Allmann, R., Acta Cryst. (1968), B24,972); Cavani et al., "Catalysis Today", 11, 173(1991) and references therein).
The most common approach to synthesis of hydrotalcites is by co-precipitation of the two cations under conditions of supersaturation (U.S. Pat. Nos. 4,165,339, 3,879,523 and references therein). They are also synthesized by reacting activated magnesia with an aqueous solution of sodium aluminate, carbonate, and hydroxyl ions (U.S. Pat. No. 4,904,457). It is well known that hydrotalcites prepared by the above procedures have a hexagonal plate-like crystal habit (Reichle, W. T., Chemtech, 1986, 58). When crystallized at room temperature, the crystallites have a diameter of approximately 0.01 to 0.1 microns and can be grown to about 1 to 5 microns by hydrothermal treatment. In all cases, the ratio of the diameter to thickness of the hexagonal crystals in such synthetic materials of the prior art is in the range of about 5 to 10. Scanning and transmission electron microscope (TEM) pictures of hydrotalcite with the hexagonal plate-like crystal morphology are shown in FIGS. 1a and 1b, respectively.
The reaction of a basic magnesium compound having a needle like structure with a suitable aluminum compound under basic conditions results in the synthesis of hydrotalcites with a needle like morphology (U.S. Pat. No. 4,351,814).
The term "hydrotalcite-like" is recognized in the art. It is defined and used in a manner consistent with usage, herein in the comprehensive literature survey of the above-referenced Cavani et al article.
For a catalyst to have good mechanical strength, the crush strength of a typical 1/16' extrudate would have to be at least in excess of 0.5 lb/mm, preferably in excess of 1 lb/mm and most preferably in excess of 2 lb/mm. The formed shapes prepared from the previously known synthetic hydrotalcites have strength of less than 0.1 lb/mm. U.S. Pat. No. 4,656,156 has mentioned that "Activated hydrotalcite is difficult to form in shapes, such as spheres, pellets and extrudates which are commonly used for adsorption and for catalyst substrates" (page 4, lines 7-10), and these hydrotalcites are clearly not suitable as-binders for other inorganic materials.
The object of the present invention is to provide novel hydrotalcite-like materials which will act as excellent binders for oxide, hydroxide and other inorganic materials used as catalysts, catalyst supports, and adsorbents. It is also the object of the invention to provide a method for synthesizing these hydrotalcites via a method which makes them suitable for use as binders. A further object of the invention is to describe a method for using these binders with different inorganic materials to form shapes with good mechanical strength.